This invention relates to air-permeable and fluid-impermeable backsheets made of polyolefin plastic sheet for use in body fluid absorbent articles such as disposable diapers, sanitary napkins and the like.
It is well known to use a polyolefin plastic sheet such as a polyethylene sheet as liquid-impermeable sheet which covers absorbent cores of body fluid absorbent articles. Use of a polyethylene sheet, which is made transparent or translucent, allows the absorbent cores stained with body fluid to be identified through observation of the sheet, and such a sheet is convenient for judgement of the time at which the body fluid absorbent articles thus stained with body fluids should be exchanged with fresh ones. However, such a well known sheet is relatively poor in its air-permeability and therefore the body fluid absorbent articles employing such a backsheet tend to become stuffy. It is also well known to stretch polyolefin plastic sheet such as a polyethylene sheet containing fine particles of inorganic filler, such as calcium carbonate, at a desired draw ratio in order to form air-permeable fine pores and to use such sheet as an air-permeable and liquid-impermeable backsheet as disclosed, for example, in Japanese Patent Application Disclosure Gazettes Nos. 1985-199037 and 1987-167332. These conventional techniques are certainly convenient in preventing the body fluid absorbent articles from becoming stuffy during use thereof owing to improved air-permeability. However, while such a sheet of prior art exhibits a certain transparency before stretching treatment, once it has been stretched, the filler particles are apt to rise to the surface and to form fine projections which, in turn, roughen the sheet surface and thereby increase its optical diffusivity. Consequently, the transparency of the sheet is lost, making it difficult for users to identify that an absorbent core has been stained with body fluids.
To solve these problems, it is a principal object of this invention to provide a backsheet having desired transparency as well as desired air-permeability and being suitable to be used as components of body fluid absorbent articles by subjecting air-permeable polyolefin plastic sheet containing inorganic filler particles and having its surface once roughed by stretching to a thermo-embossing treatment so as to provide partially transparent and smooth surface.